1. Field of Invention
The invention relates to a dynamic random access memory structure and, in particular, to a dynamic random access memory structure with checkerboard-like deep trench patterns to increase the density of devices.
2. Related Art
Dynamic random access memory (DRAM) devices are widely used in integrated circuits (IC) of the electronic industry for binary data access. As the densities of the semiconductor circuits become higher, the sizes of semiconductor devices also have to decrease. Therefore, the capacitor design in DRAM in recent years turns to three-dimensional in order to satisfy the strict requirement in current leaking that happens to small-size devices. Some even design the DRAM memory unit cell into one that consists of vertical transistors in deep trenches (DT) and DT capacitors in order to effectively increase the device density.
Although vertical transistors and DT capacitor structures can increase the memory unit cell density in the DRAM, the DT patterns are further designed to have a mirror-type symmetric structure (see FIG. 1) in order to further save the layout space.
As shown in FIG. 1, the substrate 100 is formed with an active area 102, DT patterns 104, and bit line contact holes 106. The vertical arrangement of the DT patterns 104 is in the word line direction, while the horizontal arrangement is in the bit line direction. A shallow trench isolation region 108 is provided between each two active areas 102 as electrical insulation. A feature of this conventional structure is that a memory unit cell 111 and its neighbor memory unit cell 112 have a mirror-type DT pattern 104 to form a mirror-type device symmetric structure. The memory unit cells 111, 112 share one contact hole 106.
Therefore, the DRAM device with mirror-symmetric DT patterns uses the feature that two neighboring memory unit cells share one contact hole 106 to effectively save layout space. However, some of the DT patterns 104 are so close that are connected to form short-circuit defects during the manufacturing process, deteriorating the quality of the devices. This type of DT short-circuit defects 110 is more serious as the aspect ratio of the DT patterns 104 increases.
Therefore, how to let the memory unit cells keep sharing contact holes and at the same time prevent the DT short-circuit defects from happening is important for the production of DRAM devices with higher densities.